Secondary cells comprise a rolled-up electrode unit 2 housed in a cell can 1, for example, as shown in FIG. 11. The cell can 1 comprises a cylinder 15 having a bottom, and a lid 11 fixed to an opening portion of the cylinder 15 with an insulating member 12 fitted therein around the lid. A positive electrode terminal 18 having a safety valve 14 inside thereof is attached to the lid 11 and provides a positive electrode terminal assembly 13. The bottom of the cylinder 15 provides a negative electrode terminal assembly 17.
The rolled-up electrode unit 2 comprises a positive electrode 21, separator 22 and negative electrode 23 which are each in the form of a strip. The positive and negative electrodes 21, 23 are lapped over respective separators 22, displaced from the separator widthwise thereof, and rolled up into a spiral form. At one of axial opposite ends of the rolled-up electrode unit 2, an edge of the positive electrode 21 projects outward beyond the corresponding edge of the separator 22, and at the other end of the unit, an edge of the negative electrode 23 projects outward beyond the corresponding edge of the separator 22. Current collector plates 9, 16 are arranged at the respective opposite ends of the electrode unit 2 and each welded to the projecting edge of the positive electrode 21 or negative electrode 23.
The current collector plate 9 joined to the edge of the positive electrode 21 comprises a flat disk 93 and a lead portion 91 in the form of a strip projecting therefrom. The lead portion 91 is welded at an outer end thereof to the lid 11. On the other hand, the current collector plate 16 joined to the edge of the negative electrode 23 is in the form of a disk, provided on the bottom wall of the cylinder 15 and welded to the bottom wall. The rolled-up electrode unit 2 generates electric power, which can be delivered to the outside through the positive electrode terminal assembly 13 and negative electrode terminal assembly 17.
For such secondary cells to deliver an improved output, the current path through which the electric power generated by the rolled-up electrode unit 2 is delivered to the outside must be minimized in electric resistance, i.e., in internal resistance. Especially the nonaqueous electrolyte secondary cells for use as power sources for electric vehicles need to be reduced in internal resistance to the greatest possible extent so as to be of high capacity and to deliver a great output.
However, the secondary cell having the current collector structure shown in FIG. 11 has the problem that since the edge of a current collector forming each of the positive electrode 21 and the negative electrode 23 of the electrode unit 2 is small in area, the area of contact of the current collector edge with the current collector plate is small, consequently giving increased internal resistance to the cell.
To realize reduced internal resistance, the present applicant has contrived the current collector structure shown in FIG. 6 (see JP-A No. 2001-256952). As shown in FIG. 6, the current collector plate 5 to be disposed on the positive electrode side of a rolled-up electrode unit 4 comprises a flat body 51 having a center hole 54. The body 51 is integrally provided with a plurality of circular-arc protrusions 52 extending radially from the center hole 54 and protruding toward the electrode unit 4. A plurality of raised pieces 53 are formed by slitting between each pair of adjacent circular-arc protrusions 52, 52 to project toward the electrode unit 4. The body 51 further has a lead portion 55 in the form of a strip formed integrally therewith at an end portion. The current collector plate 50 to be disposed on the negative electrode side of the electrode unit 4 also has the same construction as described above.
In the process for fabricating the secondary cell having the current collector structure described above, the current collector plate 5 for the positive electrode is pressed against current collector edge portions 48 formed at one end of the rolled-up electrode unit 4 as shown in FIGS. 7 and 9. This causes each circular-arc protrusion 52 of the current collector plate 5 to bite into the current collector edge portions 48 of the unit 4 as shown in FIG. 8, forming a joint face in the form of a cylindrical face between the protrusion 52 and the current collector edge portions 48. Each raised piece 53 of the current collector plate 5 bites deep into the current collector edge portions 48 of the electrode unit 4 as shown in FIG. 10 and is brought into pressing contact with the current collector edge portions 48.
In this state, a laser beam is projected onto the inner surface of the circular-arc protrusion 52 of the current collector plate 5 as indicated by arrows in FIG. 8 for laser welding. As a result, the protrusion 52 of the current collector plate 5 and the current collector edge 48 of the electrode unit 4 are welded to each other over a large area of contact, with the raised piece 53 shown in FIG. 10 held in pressing contact with the current collector edge 48. The current collector plate 50 for the negative electrode is also similarly welded to the electrode unit 4.
With the secondary cell described, each circular-arc protrusion 52 is welded to the current collector edge portions 48 over large contact areas of the current collector plate 5, 50, and each raised piece 53 is effectively in contact with the current collector edge portions 48 by biting into these current collector edge portions 48 in edge areas other than the weld joint areas. This results in reduced resistance of contact between the current collector plate 5 and the rolled-up electrode unit 4. Additionally, current is collected from the entire area of the current collector edge 48 by the raised pieces 53 formed on the current collector plate 5 to achieve high current collection efficiency.
The process for fabricating the secondary cell having the above current collector structure includes the step of joining the current collector plate 50 for the negative electrode to the bottom wall of the cylinder 15 from outside the cylinder 15 by laser welding, with the current collector plate 50 placed on the cylinder bottom portion.
However, the current collector plate 50 has circular-arc protrusions 52 protruding toward the electrode unit 4 as shown in FIG. 6 and slits (not shown) for forming the raised pieces 53, with noncontact regions provided locally between the surface of the current collector plate 50 and the bottom wall of the cylinder 15. If the noncontact regions are subjected to laser welding, the cylinder 15 is therefore likely to melt to produce holes at these regions.
Accordingly, an object of the present invention is to provide a secondary cell wherein an electrode unit is satisfactorily in contact with current collector plates to ensure high current collection efficiency and which can be fabricated by welding the current collector plate to the bottom wall of a cell can without the likelihood that holes will be formed in the cell can.